1. Field of the Invention
The present invention relates to a torque sensor for detecting torque generated in a rotation shaft, and particularly it is directed to attain the torque sensor in which not only labor at the time of assembling can be simplified but also reliability can be improved.
The present application is based on Japanese Patent Applications Nos. Hei. 10-60404 and 10-60406, which are incorporated herein by reference.
2. Description of the Related Art
As related art of this type, for example, there is such a technique as disclosed in Japanese Patent Publication No. Hei. 8-240491. In this torque sensor, first and second rotation shafts disposed coaxially are connected to each other through a torsion bar. Further, a cylindrical member made of an electrically conductive non-magnetic material is integrated with the second rotation shaft in the direction of the rotation thereof so as to enclose an outer circumferential surface of the first rotation shaft. At least a portion of the first rotation shaft which is enclosed by the cylindrical member is made of a magnetic material. Grooves extending axially are formed in the enclosed portion. Windows are formed in the cylindrical member so that the state of overlapping of the windows with the grooves changes in accordance with the position of rotation of the cylindrical member relative to the first rotation shaft. Further, coils are disposed so as to enclose the portion of the cylindrical member in which the windows are formed. Torque is detected on the basis of inductance in the coils. Accordingly, there is obtained an effect that not only high-accurate torque detection is performed by a simple structure but also reduction in apparatus size is attained.
Certainly, such a related-art torque sensor as disclosed in Japanese Patent Publication No. Hei. 8-240491 can show the aforementioned effect. However, as a result of earnest researches made by the present inventors, it was found that the structure of fixture of the cylindrical member to the rotation shaft is important to keep the reliability of the torque sensor high.
That is, in the case of a torque sensor as disclosed in Japanese Patent Publication No. Hei. 8-240491, the cylindrical member is made of aluminum, or the like, whereas the rotation shaft for fixing the cylindrical member thereon is made of iron, or the like. Accordingly, the thermal expansion coefficients of the two are often different from each other. As a result, with such a simple structure in which the cylindrical member is press-fitted into the rotation shaft, the power for holding the cylindrical member to the rotation shaft may vary in accordance with the temperature so that the holding power cannot be kept. Further, when the holding power is reduced, both the rotational position and axial position of the cylindrical member relative to the rotation shaft are displaced so that detection accuracy is lowered.
As for other torque sensors, for example, there are those which are disclosed in Japanese Patent Publication Nos. Hei. 4-47638, 8-5477, and so on. In these torque sensors, torque acting on a rotation shaft is made to be reflected in the change of impedance of a coil so that the impedance change is detected to thereby detect torque. That is, the coil is disposed so as to enclose the rotation shaft, so that the impedance of the coil is changed on the basis of the change in magnetic or mechanical structure in accordance with torque of the rotation shaft. Accordingly, if a voltage between terminals of the coil is measured to thereby detect the impedance change, torque generated in the rotation shaft can be detected. Further, in the related-art torque sensors, in order to cancel the impedance change of the coil caused by factors such as temperature, etc. other than torque, two coils are disposed so that impedance changes caused by torque become reverse in direction to each other. A bridge circuit containing these two coils is formed so that torque is detected on the basis of the difference between two outputs of the bridge circuit. That is, even in the case where impedance changes of the coils are caused by factors other than torque, the impedance changes caused by such factors can be canceled with each other when the difference between the output voltages of the bridge circuit is obtained because the impedance changes due to such factors are generated in one and the same direction in the two coils.
Here, the aforementioned torque sensor has a coil bobbin on which each coil is wound. Pins for connecting coil end portions to a substrate are fixed to the coil bobbin. That is, the coil bobbin is generally made of plastics. For example, two metal pins corresponding to the coil are fixed to a terminal portion of the plastic coil bobbin. The end portions of the coil are wound on the bases of the pins. Top ends of the pins are inserted into the substrate so that electrical conduction is made between the substrate and the coil. Further, the coil bobbin is generally received in a coil yoke which is fixed to the housing to form a magnetic circuit.
In the aforementioned configuration, when a coil bobbin is put in a coil yoke, a gap may be produced between the coil bobbin and the coil yoke in accordance with the machining accuracy, or the like, of various parts. In some case, combination is made so that the coil bobbin may move axially relative to the coil yoke. Then, when the coil yoke is fixed to the housing, the positions of pins are not determined. This caused inconvenience when pins were inserted in predetermined positions of the substrate, and became hindrance against automatization, or the like. Further, even after the pins were fixed to the substrate by means of soldering, or the like, the coil bobbin moved axially, so that load was imposed on a joint portion between each pin and the substrate and the probability of occurrence of joint failure, or the like, was relatively high.